Well getting characters to lvl 60+ and weapons to 70 is a good time to end it. After this its not possible to even progress. Mostly due to a book asking you to upgrade your relics to way to high lvl and if you wanted to do this, you would have to use all the good ones you have just to pass it.
Getting MC up is easy as you get free crystals to lvl it up as a reward, but rest of team not so much. You are locked to a boss battle or upgrade books and you gota plan and wait to use up resin smart.
Endgame realy is just farming elite mobs all over the map and hoping you get good drops, as dungeons cost a lot. Heck farming dungeon relics is last thing you should do once you get every character to lvl 70 with 70 weapon.